From the Super Mario Wiki, the Mario encyclopedia
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The stone keys are special artifacts in Paper Mario: The Thousand-Year Door. They are Moon Stone and the Sun Stone, two gems shaped like sun and moon, and they are required to open up the way to Hooktail's Castle. Mario learns of these items from Kroop, the mayor of Petalburg during his search for the Diamond Star located in Hooktail's Castle. Kroop tells Mario that the stone keys lie in a ruined stronghold east of the village and that he has to go there in order to enter the castle. Mario finds the relics guarded by a Thwomp, a horde of Fuzzies and a Gold Fuzzy. After he obtains the two items, he can go to the stone formation in Petal Meadows and use them, as well as Koops' abilities, to make a pipe to Hooktail's Castle appear.